Process for making secondary emission electrodes



PROCESS FOR MAKING SECONDARY EMISSION ELECTRODES Filed April 6} 1946 Feb. 26, 1952 M. ARDIT] ETAL 2 SHEETS-SHEET l FIG. 2

AFTER BOMBARDMENT BEFORE BOM BARDMENT VELOCITIES IN VOLTS WALTER URBAN A TTORNEY Feb. 26, 1952 M. ARDITI ETAL PROCESS FOR MAKING SECONDARY EMISSION ELECTRODES Filed April 6, 1946 FIG. 3

VELOCITIES 2 SHEETS-SHEET 2 BEFORE BOMBARDM ENT [0 r TL AFTER BOMBARDMENT a b a BEFORE BOMBARDMENT VELOCITIES IN VOLTS l I l I O 500 I000 I500 Z000 AFTER BOMBARDMENT INVENTORS IN VOLTS MAURICE ARDITI BY WALTER URBAN- A TTORNEY Patented Feb. 26, 1952 UNITED STATES PATENT OFFICE PROCESS FOR MAKING SECONDARY EMISSION ELECTRODES Delaware Application April 6, 1946, Serial No. 660,106 4 Claims. (Ci.- 316-22) The present invention relates to electronemitting electrodes and particularly to secondary emission electrodes.

Proposals have been made with respect to such electrodes for increasing their secondary electronic emission coeillcient (ratio of a number'of secondary electrons emitted to the number of primary electrons received). For example in the French Patents 862.488; 862,489 and 862,490 to Maurice Arditi and in pending U. S. application Serial No. 482,898, filed April 13, 1943 also by Maurice Arditi entitled Electron Emission Electrodes," (now abandoned), various methods of oxidizing alloys according to a selective oxidation process" were proposed for increasing said coefllcient. According to this selective oxidation process a superficial oxide layer showing good secondary electronic emission properties is obtained.

In accordance with a feature of the present invention I have discovered that by bombarding secondary emission electrodes, such as for example those treated in accordance with the aforementioned patents and copending patent application, with a high density electron beam having a relatively high velocity, for a given time, the coefllcient of secondary emission'is greatly im proved.

An object of the present invention is the provision of a method for increasing the secondary electronic emission coefficient of secondary emission electrodes, particularly of the type treated in accordance with my aforementioned patents or patent application.

Other and further objects of the present invention will become apparent and the invention will be best understood from the following description of examples thereof, reference being bad to the drawings in which:

Fig. 1 is-a schematic diagram of an electron discharge device in which a secondary emission electrode is bombarded in accordance with the present invention and which device may thereafter be employed as a trigger tube or for various other purposes; and

Figs. 2, 3 and 4 are sets of curves indicating the results obtained from various secondary emission electrodes before and after bombardment in accordance with the present invention.

In preparing a secondary emission electrode in accordance with a feature of the present invention, the electrode which may consist of one of the alloys mentioned hereinafter and also mentioned in the aforesaid patents or copending application, may be oxidized according to the processes mentioned in said patents or application. The oxidized electrode is then mounted within an envelope which envelope is evacuated, and the electrode is then bombarded with a. high density, high voltage electron beam for a given period of time. The electron density of the beam 2 during this bombardment is substantially higher than that to which the electrode is subjected during normal operation. As a matter of fact, if a secondary emission electrode were subjected to such a high density beam during operation thereof, it would after a given time lose the improved emission qualities characteristic of those electrodes treated according to the present invention. During the bombardment of one particular group of secondary emission electrodes, the current employed was about milliamperes while normal operation current was about 1 to 2 milliamperes. The bombardment was carried out for a few seconds. When using smaller currents it has been found that the bombardment must be continued for a longer time, but under all circumstances the current employed was substantially greater than normal operation current. In such bombardment of particular samples of secondary emission electrodes, voltages with respect to cathodes ranging from about 500 volts to 2.000 volts were employed on the secondary emission electrode, and it was found that suchvoltages results appear to fall off. This may be due to the penetration of the electrons of the beam deeply into the electrode where their effects upon surface emission are slight.

In bombarding a secondary emission electrode in accordance with the present invention it is desirable that the electron velocity of the electrons bombarding said electrode be about 500 volts or greater; thus in'processing the successive secondary emission electrodes of an electron multiplier it is desirable that the potential difference between successive electrodes be about 500 volts or greater, since the initial velocity of electrons from a secondary emission electrode is of the order of only 0 to 20 volts which is much too small for bombardment in accordance with the present invention.

The following is an example of a beryllium copper electrode which consists of about 2% beryllium, 98% copper and 0.2% cobalt. This electrode is cleaned and sealed into a hard glass tube, with a vigorous stream of nitrogen passing through the tube while sealing the tube to prevent oxidation of the electrode material. After annealing the tube, which is sealed. it is evacuated by a suitable exhaust system which has provision for admitting oxygen. The tube is exhausted until the pressure is down to 15 microns or less, and the tube is heat treated with radio frequency current until all the gas has been liberated from the secondary emission electrode or approximately 600-700, and held for six (6) seconds only at this temperature. The oxygen is then thereupon immediately sucked out and when the tube is cooled sufficiently to be handled, it is tipped off and held in this condition until the electrode is ready to be mounted in an assembly. The foregoing completes the selective oxidation" in accordance with said patents and pending application.

In bombarding the secondary emission electrode in accordance with one aspectof the present invention, it may be mounted in an evacuated tube, as for example the tube in which it is to be used, which may for example contain an electron gun I, whose cathode 2 is aged in any suitable manner (Fig. 1). The secondary emission electrode 3 is bombarded for a few seconds with a current of about 60 milliamperes, the peak voltage on the secondary electrode being between 500 and 2,000 volts from a suitable source of potential 4. The tube may include a collector electrode 5, which may be used in collecting the electrons during bombardment, in testing the secondary emission, and in normal operation.

Referring now to Fig. 2, the ratio N between the number of secondary electrons emitted and the number of primary electrons received is plotted along the vertical coordinate while the velocity of the primary electrons expressed in volts is plotted along the horizontal coordinate. Curve 6 indicates the results obtained before bombardment and curve I indicates the result obtained after bombardment in accordance with the present invention.

An alloy known as alloy 61 SO, which is comprised of 25% copper, .60% silicon, 25% magnesium, 25% chromium, with the remainder being aluminum, was likewise similarly oxidized and bombarded. The curves 8 and 9 of Fig. 3 indicate the. results obtained before bombardment and after bombardment respectively.

A silver magnesium alloy, which consists of silver and magnesium, oxidized according to said patents or said application and consisting for the most part of silver and an amount of magnesium in the alloy which is not critical and ranges between less than 1% to about 15% and which is oxidized (see Journal of Applied Physics, volume 12, September 1941, page 696) was similarly subjected to bombardment in accordance with the present invention. Curves l and II of Fig. 4 indicate the results obtained before and after bombardment, respectively.

While it will be appreciated that the results obtained are completely independent of any theory, my present theory explaining why such a high ratio was obtained as a result of bombardment in accordance with my invention, is substantially this: the selective oxidation process, described in my aforementioned patents and copending application produces a thin film of the oxide of the more active metal, like beryllium oxide on the surface of the beryllium copper electrode. When said electrode is bombarded by electrons, the oxide may become dissociated, some free metal diffusing into the surface and causing the high ratio of secondary emission. It may be stated as an hypothesis that maximum secondary emission is obtained when a certain equilibrium exists between the amount of free metal and the oxide, and this may explain why the ratio goes down when the electrode is bombarded too much. This theory may be correlated with the theory of semi-conductors in explaining some of the results obtained. due to the fact that the free metal dis- 4 trlbuted in the oxide makes the surface of said elecrode a type of semi-conductor.

While I have described the specific details of carrying out my process for certain secondary emission electrodes under certain specific conditions, it will be appreciated both that the values stated may be changed, such as for example that much smaller currents may be employed or that much larger currents may be employed, or other secondary emission electrodes of difierent chemical composition may be similarly processed. However, under all circumstances the currents employed are larger than normal operating currents.

- We claim:

1. In the method of preparing a secondary emission electrode comprised of an alloy for operation in an electron discharge device in which it is impinged upon by an electron stream having given normal operating densities, the steps comprising mounting the electrode in an evacuated container, oxidizing the surface of said electrode, and bombarding the oxidized electrode with an electron stream of high velocity having a relatively greater density than any of said normal operating densities.

2. The method according to claim 1, wherein the bombarding electrons are given a velocity of a minimum value of the order of at least 10 times the initial velocity of electrons from said secondary emission electrode.

3. In the method of preparing a secondary emission electrode comprised of an alloy for operation in an electron discharge device in which it is impinged upon by an electron stream having given normal operating densities, the steps comprising mounting the electrode in an evacuated container, oxidizing the surface of said electrode. and bombarding the electrode for a limited period of the order of a few seconds duration with an electron stream of high velocity having a relatively greater density than any of said normal operating densities and having an electron velocity of about 500-2000 volts with respect to the cathode v 4. In a process for the manufacture of a vacuum tube electrode having an abnormally high secondary emission coefl'icient comprising selecting an alloy containing a predominant proportion of a metal selected from the class consisting of aluminium magnesium and beryllium, forming an electrode from said alloy, and heating said electrode in the presence of oxygen at a temperature such that a polished surface of the alloy would become slightly granular in appearance whereby the electrode is superficially oxidized, the improvements that comprise bombarding said superflcially oxidized electrode for a period of a few seconds with a high-density high-voltage electron stream, the density of said stream being substantially higher than that to which the electrode will be subjected in its intended use.

MAURICE ARDITI. WALTER D. URBAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS France Dec. 9, 1940 

1. IN THE METHOD OF PREPARING A SECONDARY EMISSION ELECTRODE COMPRISED OF AN ALLOY FOR OPERATION IN AN ELECTRON DISCHARGE DEVICE IN WHICH IT IS IMPINGED UPON BY AN ELECTRON STREAM HAVING GIVEN NORMAL OPERATING DENSITIES, THE STEPS COMPRISING MOUNTING THE ELECTRODE IN AN EVACUATED CONTAINER, OXIDIZING THE SURFACE OF SAID ELECTRODE, AND BOMBARDING THE OXIDIZED ELECTRODE WITH ANELECTRON STREAMS OF HIGH VELOCITY HAVING A RELATIVELY GREATER DENSITY THAN ANY OF SAID NORMAL OPERATING DENSITIES. 